The effect of silver coating on magnetic properties of oxygen-stabilized tetragonal Ni nanoparticles prepared by chemical reduction

Ni–nickel oxide and Ni–Ag nanoparticles with core–shell morphology have been synthesized by a simple two-step chemical route. It involves the reduction of Ni 2+ cations to Ni particles with sodium borohydride (NaBH 4 ) in aqueous solution, followed by coating part of the sample with a surface layer of silver. The coating was accomplished via a simple transmetallation reaction, , in aqueous AgNO 3 solution at room temperature. Particles left uncoated have a thin spontaneous surface oxide layer of nickel oxide, presumably non-stoichiometric, as a passivating layer. From x-ray diffraction patterns, the Ni in as-prepared samples, both coated and uncoated, has been identified as being in a tetragonal crystal structure, different from its usual fcc form. This structural modification is due to the presence of interstitial oxygen atoms in the Ni lattice, and results in appreciably modified magnetic properties in this new phase of Ni, for example a linear non-hysteretic magnetization response with applied field at room temperature. A comparative magnetic study of the uncoated and silver coated Ni particles exemplifies the role played by the Ag shell in modifying the magnetic properties of the coated sample. This is evident from the smaller coercivity and remnant magnetization in the hysteresis loop at 5 K in the case of the Ag coated as-prepared sample, larger magnetic moment but smaller saturation magnetization and susceptibility of the Ag coated air annealed sample, and higher blocking temperature for this sample in comparison to the uncoated counterparts. These features have been coherently explained on the basis of the structural modifications induced in these samples by Ag coating.